1. Field of the Invention
The present invention relates to optimizing cooperation areas and cover shifts in cellular communications network systems, for example LTE or LTE-A systems.
2. Related Background Art
Prior art which is related to this technical field can e.g. be found in:    [1] W. Mennerich and W. Zirwas, “User centric coordinated multi point transmission,” in Proceedings of IEEE 72nd Vehicular Technology Conference (VTC2010-Fall), Ottawa, Canada, 2010.    [2] W. Mennerich and W. Zirwas, “Reporting Effort for Cooperative Systems Applying Interference Floor Shaping,” in Proceedings of IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC '11), September 2011, Toronto, Canada.
The following meanings for the abbreviations used in this specification apply:    CA cooperation area    CoMP coordinated multi-point    CSI channel state information    eNB evolved node B    ID identity    LTE long term evolution    LTE-A LTE advanced    RSRP reference signal received power    SON self optimizing networks    Tx transmit    UE user equipment
In particular, the present invention relates to SON and CoMP as features of LTE-A and beyond. A CoMP transmission is a coherent multi-cell transmission where several data streams are jointly transmitted over the same radio resources from multiple cells to multiple terminals (UEs) within these cells. Under the assumption of complete knowledge of channel state information (CSI) for all jointly used links joint pre-coding of all links will be interference-free decodable.
Cooperation between all cells is not realizable and must be limited to a moderate number of cells, so-called cooperation areas (CAs), since a considerable amount of signaling information has to be exchanged among all cooperating cells as well as CSI for all relevant radio channels has to be fed back by the UEs, where both need to be kept to a manageable amount.
Users benefit the most from CoMP in case of so-called “user centric” cooperation areas, i.e. the CAs should include the strongest cells for each UE as described in reference [1]. However, several disjoint predefined CAs lead to the situation that for several UEs the strongest cells do not belong to one single CA, i.e. UEs have individual sets of their strongest cells. In particular, those UEs at the fringe of the disjoint CAs have a high likelihood for strong cells belonging to a neighboring CA. Since larger CA sizes are not feasible, shifted and overlapping CAs—so called cover shifts—operating on different frequency layers have been proposed e.g. in reference [2]. The determination of the cover shifts goes in hand with optimal antenna tilting e.g. according to the Tortoise concept.
As long as the network layout is regular (hexagon grid) and user traffic is homogeneously distributed both the cooperation areas and the cover shifts can be exactly planned in advance as shown e.g. in FIG. 1. In FIG. 1, different shifts of CAs are marked by different grey-levels: Edges of triangles mark cooperating cell sites, different grey-levels within triangles denote the sub-band a CA is using. From left to right, figures show one, two, three and five shifts.
However, these regular overlapping CA shifts are not applicable in real network deployments where cells look much more unshaped due to deployment and propagation irregularities. In addition, a predetermined partitioning of the resources among cover shifts is per se not adapted to the user and traffic distribution. The planned version is based on assumptions and if the assumptions do not meet real user and traffic distributions the wrong partitioning leads to unwanted capacity shortages in some cover shifts while others have more than needed.
It is to be noted that different cover shifts are defined for orthogonal resources like e.g. different frequency bands or time slots, and even different codes. This resource allocation has to be done ideally network wide as otherwise there will be inter-cover-shift interference at least for that part of the resources where network wide orthogonality is violated.
An additional problem arises if to a running system additional eNBs or pico stations are being added, which in turn will have to be integrated into the already defined cover shifts. In worst case this might lead in terms of optimal setup to a complete reorganization of the cover shifts.